The present invention relates to a secret or privacy signal transmission system which sends out an audio signal or like analog signal after transforming it into a privacy signal.
A transmission system of the type described is not only capable of keeping eavesdroppers from communications between two parties but is also capable of eliminating leak message leaks when the parties are accidentally connected to a third party due to a transmission line failure.
Privacy signal transmission systems heretofore proposed may generally be classified into two types, i.e., a spectrum inversion type and a type which allows messages to be exchanged using privacy codes (e.g. key codes or PN codes) and digitally processing them for privacy. The spectrum inversion type system is not a perfect privacy implementation, however, since it inherently allows sound volumes to be identified and even part of voice overhead thereby allowing on unauthorized party to understand the communication. It is also deficient in the conversation leak occur in the event of common channel interference in a radio system having the same type of independent recivers The privacy code type system, on the other hand, is free from the possibility of eavesdropping or leak, since conversations are exchanged between only specific individuals who share common privacy codes (e.g. key codes or PN codes). Nevertheless, this brings about a problem in reproduction quality because transmission of a signal at a low rate, which is required to limit the transmission band in digital signal transmission, introduces quantization noise in the reproduced voice or substantial wave distortion due to quantization. If no limitation is imposed on the transmission frequency band to allow signals to be fed at a high rate, an encryption circuit will have to be scaled up and, therefore, expensive, even though the waveform distortion due to quantization noise may be suppressed.
Reference may be made to the following documents for details of the privacy code type signal transmission system:
(1) Whitfield Diffie et al. "New Directions in Cryptography", IEEE TRANSACTIONS OF INFORMATION THEORY, Vol. IT-22, No. 6, p.p. 644-654, November 1976.
(2) Horse Feistel et al. "Some Cryptographyic Techniques for Machine-to-Machine Data Communication", PROCEEDINGS OF THE IEEE, Vol. 63, No. 11, p.p. 1545-1555, November, 1975.
(3) U.S. Pat. No. 3,784,743 issued Jan. 8, 1974.